In 1965 Edward F. Gurnee and Fernandez Reet Teoste first observed and studied organic electroluminescence (U.S. Pat. No. 3,172,862). Later on, Tang in Eastman Kodak disclosed double-layer structure OLED (organic light emitting diode, U.S. Pat. No. 4,356,429; Appl. Phys. Lett. 1987, 51, 12, 913). This diode was based on employing a multilayer structure including an emissive electron-transporting layer (fabricated from Alq3 (q=deprotonated 8-hydroxyquinolinyl)) and a hole-transport layer of suitable organic materials. Afterward, research on materials used in OLED becomes a hot research topic. OLED possesses many advantages such as: low operating voltage; ultra thin; self emitting; good device efficiency; high contrast and high resolution which suggest the possible use of OLED in flat panel displays and lighting.
There are two classes of emitting materials for OLED application: fluorescent and phosphorescent materials. Phosphorescent materials become the major trend for emitting materials development since 75% excitons produced from OLED are in triplet, only 25% excitons are in singlet. It means the maximum device efficiency for phosphorescent materials are 3 times higher than fluorescent materials.
Platinum is one of the transition metals from emissive complexes with organic ligands, which have high emission quantum efficiency and good thermal stability. With these advantages, platinum(II) complexes were used as emitting materials in high performance OELDs. (Applied Physics Letters (2007), 91(6) 063508; Chemistry—A European Journal (2010), 16(1), 233-247) Among the platinum complexes used in OLED applications, pure green emitting materials with stable chemical structure are rare.
For the stability of the platinum(II) complexes, the binding energy between the ligand and platinum(II) center gets higher when the number of coordination positions in the ligand increases; that is, the binding energy between the ligand and platinum(II) center is the highest in tetradentate ligand platinum(II) complexes. Moreover, the addition of extra atom(s) between the aromatic coordination position to break the conjugation of the ligand may weaken the stability of the ligand and eventually weaken the stability of the complexes. Green emitting platinum(II) materials with bidentate ligands, tridentate ligand or tetradentate ligand with extra atom(s) between the aromatic coordination position to break the conjugation is not as good as a conjugated tetradentate ligand system.
However, most of the conjugated tetradentate ligand systems are not able to have pure green emitting materials due to their intrinsic properties such as the band gaps are limited by the MLCT transitions and the emission spectra are vibronically structured. (see U.S. Pat. No. 6,653,654; U.S. Pat. No. 7,361,415; U.S. Pat. No. 7,691,495). For these reasons, stable green emitting platinum(II) material is difficult to develop.